Puzzle 140 ("Medium Nikoli" series, Chained Block)

Back at it again with the Nikoli series!  This time we tackle Chained Block.  Frequent readers of this blog will recall that this is not my first rodeo with this genre.

This puzzle is not as smooth as some of my others -- there are one or two places where a tiny amount of lookahead is required, I think.  However, I'm still putting it in this series because of its use of question mark clues, which denote blocks of unknown size.  Most non-Nikoli sources don't use question mark clues to their full extent, so I wanted to make something interesting happen with them in this puzzle.  This is the result.

Difficulty "Extra", and it's not even close.

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